Methods of forming expandable polymer beads or pellets which are suitable for forming shaped foamed polymer articles are known in the art. For example, the Ikeda et al U.S. Pat. No. 4,080,344 discloses methods for the production of expandable ethylenically unsaturated polymer particles from ethylenically unsaturated polymer particles by suspending the particles in water and impregnating the particles with an organic expanding agent having a boiling point lower than the softening point of the polymer particles. Ikeda et al include a suspension stabilizer comprising calcium carbonate particles having resin acid-treated surfaces in the particle-water suspension.
The Sakata et al U.S. Pat. No. 4,550,003 generally discloses additional methods for incorporating a blowing agent such as Freon into resin particles using gas phase or liquid phase impregnation methods wherein the resin particles are impregnated in an autoclave with a liquid or gaseous blowing agent, under pressurization if desired, and in-water suspension impregnation methods in which resin particles are suspended in water and impregnated with a blowing agent. Sakata et al also disclose that expandable resin particles may be prepared by effecting polymerization in the presence of a blowing agent.
The Hall, III et al U.S. Pat. No. 5,580,503 discloses another conventional method for preparing expandable polymer beads wherein a slurry of thermoplastic minipellets containing a blowing agent is pumped through a series of impregnation vessels at a predetermined pressure and temperature and for a sufficient time to allow the blowing agent to impregnate the thermoplastic particles. The slurry is typically formed of water, thermoplastic minipellets and optionally a dispersing agent and/or a surface active agent. Additional methods for preparing expandable polymer particles or beads are disclosed in the Keneko et al U.S. Pat. No. 5,525,639, the Ueda et al U.S. Pat. No. 5,132,330, the Mori et al U.S. Pat. Nos. 5,128,380 and 4,721,731, and the Mostafa U.S. Pat. No. 4,361,656.
As is known in the art, expandable impregnated polymer beads or pellets may be used to prepare shaped foamed polymer particles. Generally, a rate limiting step in the production of shaped foamed polymer articles from expandable polymer beads or pellets is the time required to impregnate the polymer beads or pellets with a liquid or gaseous blowing agent. Accordingly, there is a continuing need to develop improved methods and/or apparatus for preparing expandable polymer particles in a reduced time. However, many prior efforts to reduce the time required to prepare the expandable polymer beads or pellets by reducing the time required for impregnating either a gaseous or liquid blowing agent have resulted in products which exhibit material degassing, i.e., loss of the impregnated blowing agent, thereby causing the material to collapse. Accordingly, there is a continuing need for improving methods and apparatus for preparing expandable polymer particles in the form of beads, pellets or the like.